1. Field of the Invention
The present invention relates to a light illumination device for illuminating an object with light rays uniform in intensity, and more particularly to a light illumination system suitable for use in an exposure device for fabricating semiconductor devices such as ICs.
2. Description of the Prior Art
Exposure devices for transferring integrated-circuit patterns on reticles or photomasks onto substrates need a light illumination device for providing an illumination by light rays having a flat or uniform intensity distribution in order to project an image of fine circuit patterns onto a substrate with a higher resolution.
One such light illumination device is known from U.S. Pat. No. 3,296,923 issued on Jan. 10, 1967 to J. R. Miles.
The light illumination device as described in the above-mentioned U.S. Patent has a collimating lens for collimating a luminous flux or light beam from an elliptical reflector mirror, first and second lenticular lenses for forming a plurality of secondary light sources from the luminous flux from the collimating lens, and a large aperture condenser lens disposed between the lenticular lenses and a substrate. This prior art light illumination device is capable of correcting luminous flux having an annular intensity distribution pattern weaker in the vicinity of the optical axis and stronger at the peripheral edge in the opening of the elliptical mirror, into luminous flux having a substantially flat intensity distribution on an object surface disposed behind the condenser lens. With the conventional device, however, the luminous flux of flat intensity distribution is available only on a plane corresponding to the object surface, and the annular intensity distribution pattern is left unremoved anywhere behind the condenser lens except for the object surface. This is because the luminous flux per se emitted from the first and second lenticular lenses have such an intensity distribution pattern.
For providing an illumination of the object with luminous flux from the known device, it is necessary to position the object extremely critically along the optical axis so as to attain a desired flat intensity distribution pattern on the object. Where the conventional system is employed, for example, as a light projection system for projecting an image of the object onto a projection surface with a projection lens, there is provided a flat intensity distribution pattern on focal points of the projection lens, that is, the object surface and the projection surface. However, an annular intensity pattern is produced at other planes, in particular, the pupil plane of the projection lens. As a result, the light projection system suffers from various difficulties such as a reduction in the resolving power of the projection lens, a reduction in the depth of focus thereof, and the like.